1. Field of the Invention
This invention relates to thermographic imaging systems, particularly those involving thermally assisted oxidation of a leuco dye to generate color.
2. Background of the Related Art
With the advent of digital image processing, thermographic imaging systems have become increasingly more important for printout systems. It is desirable that thermographic systems be essentially solvent free for environmental and other practical reasons. The present invention satisfies these needs by complexing leuco dyes with oxidizing anions in a binder to form relatively stable thermographic compositions. The compositions of the present invention overcome the need for diffusion in the oxidation step, thereby allowing rigid binders to be used.
Many processes and compositions use leuco dyes to provide optical densities in the imaged article. More specifically leuco dyes which are converted to colored forms by chemical oxidation (i.e., oxidatively triggerable leuco dyes), have been widely used in thermographic applications. Examples of leuco dyes used in color image forming systems include triarylmethanes, xanthenes, styryl dyes, and azine dyes, such as, phenoxazines, phenothiazines, and phenazines.
Acid salts of leuco dyes have been employed in electrochromic imaging systems (e.g., U.S. Pat. Nos. 3,390,994, and 4,444,626).
Acid salts of leuco dyes have also been employed in photosensitive imaging systems (U.S. Pat. Nos. 3,630,736 and 3,445,234). Leuco dye acid salts were employed in cases where the oxidized dye form was cationic. Despite claims to the utility of salts with oxidizing acids, no examples other than chlorides and p-toluenesulfonates were given. The imaging system of those patents is not thermally developed and requires the use of a hexaarylbiimidazole photosensitizer.
U.S. Pat. Nos. 4,373,020 and 4,336,323 describe thermally sensitive imaging systems comprised of a binder, nitrate, organic acid, and a leuco dye or bleachable dye, respectively.
U.S. Pat. No. 4,379,835 achieves a thermally sensitive imaging system with a black image by combining two leuco dyes with a metal nitrate, binder and organic acid. U.S. Pat. No. 4,563,415 discloses a thermally sensitive imaging composition comprised of a naphthoylated leuco dye, nitrate salt, and, optionally, a stabilizing compound and organic acid. Although the organic acid is optional, its presence greatly reduces the development temperatures required.
U.S. Pat. No. 4,423,139 discloses stabilizers for leuco dye nitrate salt thermographic systems. Imaging systems comprising an oxidation tripped leuco dye, nitrate salt, and organic acid show marked instability in the presence of even small quantities of moisture, particularly while in solution during formulation prior to coating. The compositions of the present invention overcome this instability by using a preformed oxidizing acid salt of a leuco dye thereby removing the need for added organic acid. The coating solutions of the present invention show marked improvements in stability, and importantly, reproducibility.
Japanese Pat. No. 88,058,108 teaches the use of salts of heptavalent and hexavalent molybdenum compounds and benzoyl leuco methylene blue in thermally imaged compositions. Also included are nitric acid salts of leuco methylene blue, but only when simultaneously in the form of a molybdenum complex or salt. Nitric acid, sulfuric acid, and hydrochloric acid are taught as forming useful salts with leuco methylene blue in the presence of molybdenum compounds.
Japanese Pat. No. 88,058,109 teaches the use of salts of heptavalent molybdenum compounds and benzoyl leuco methylene blue in thermally imaged compositions. Also required is an included compound containing both oxidizer and reducer moieties.
Japanese Pat. No. 88,058,110 describes the use of salts of hexavalent and heptavalent molybdenum compounds condensed to form iso- and hetero-polyacids in thermally imaged compositions.
Japanese Pat. No. 88,058,111 discloses the use of salts of hexavalent and heptavalent molybdenum compounds in the presence of blocked phenols in thermally imaged compositions. In the latter four disclosures, only blue to black images were successfully generated. This is expected, since molybdic acid oxidation of organic compounds generally leads to dark blue-black color formation. This fact makes molybdenum based oxidizers unsuitable for applications in which other colors such as red or green are desired. Systems without molybdenum compounds therein are therefore desirable.
The compositions of the present invention offer an improvement over leuco dye, nitrate salt systems of the prior art in that the preassociation of the leuco dye and oxidizing acid leads to improved thermographic sensitivity and environmental stability.